1. Field of the Invention
The present invention generally relates to an optical transmission device used in an optical communication network, and more particularly to an optical transmission device used in an optical communication network which employs a synchronous digital hierarchy.
An optical communication network has been practically used as means for providing broadband services in which a variety of data on telephone, facsimile, images and so on is integrated. The user/network interface in the optical communication network has been internationally standardized, and is known as a Synchronous Digital Hierarchy (SDH), as defined in the CCITT recommendations G707, G708 and G709, the disclosure of which is hereby incorporated by reference. A network which conforms to the SDH has been practically used as SONET (Synchronous Optical NETwork) in the North America.
2. Description of the Prior Art
First, a description will be briefly given of the SONET. The SONET is described in, for example, William Stallings, "ISDN and Broadband ISDN, Macmillan Publishing Company, 1992, pp. 546-558.
In the SONET, a multiplexed optical carrier (OC) is transmitted. The transmission device converts the optical signal (carrier) into an electric signal and vice versa. The electric signal is called a synchronous transport signal (STS). The basic bit rate of the SONET is 51.84 Mbps. The optical carrier having the above basic bit rate is expressed as OC-1. Generally, an optical carrier or signal is expressed as OC-N where N (optical carrier level N) is an integer, and a corresponding electric signal is expressed as STS-N (synchronous transport carrier level N). For example, the optical carrier OC-12 is an optical carrier or signal having a bit rate of 622.080 Mbps (=12.times.51.84 Mbps). In the SONET, signals having bit rates which are integer multiples of the basic bit rate. The optical carrier OC-12 is obtained by multiplexing 12 STS-1 signals at the byte level to thereby generate an STS-12 signal and by converting the STS-12 signal into an optical signal. Generally, the multiplexing of STS-N signals employs a byte-level interleave process.
It will be noted that the STS-3 in the SONET corresponds to a synchronous transport module STM-1 in the SDH. Similarly, the STS-12 corresponds to the STM-4.
The signal STS can be obtained by, for example, sequentially multiplexing digital signals having lower bit rates, such as DS-0 (64 Kbps), DS-1 (1.5 Mbps), DS-2 (6.3 Mbps) and DS-3 (45 Mbps).
FIG. 1 is a block diagram showing the outline of a network of the SONET. Electric signals from terminals 1 and 2 are respectively multiplexed by transmission devices 3 and 7, and resultant multiplexed signals are converted into light signals, which are then sent to transmission paths 8 formed of optical fiber cables. Repeaters 4, 5 and 6 are provided in the transmission paths 8. Particularly, the repeater 5 has a function of terminating the optical signals (the above function is called an add/drop function). As shown in FIG. 1, terms "section", "line" and "path" are defined in the SONET. The section corresponds to an optical transmission part between transmission devices, between repeaters or between a transmission device and a repeater. The line corresponds to an optical transmission part between transmission devices, between repeaters or between a transmission device and a repeater, each having the terminating function. The path indicates the end-to-end optical transmission part.
FIG. 2A is a diagram showing the frame format of the signal STS-1. As shown in FIG. 2A, the signal STS-1 consists of 810 octets, and is transferred every 125 .mu.s. The 810 octets consists of nine rows arranged in a matrix formation, each of the rows consisting of 90 octets. In other words, the signal STS-1 has a 9.times.9 matrix formation. The first three columns (three octets.times.nine rows) forms an overhead in which a variety of control information concerning transmissions. The first three rows of the overhead forms a section overhead, and the remaining six rows forms a line overhead. The control information forming the overheads is also referred to as overhead information.
FIG. 2B is a diagram showing the frame format of the signal STS-3. In the SDH, a new format is not created during the hierarchically multiplexing operation. That is, the signal STS-3 can be formed by simply byte-multiplexing the signals STS-1 including the headers thereof without forming a new header specifically directed to the signal STS-3.
FIG. 3A shows the section overhead and the line overhead, and FIG. 3B shows the path overhead. The bytes forming these overheads are well known, and a description thereof will be omitted here.
FIG. 4 is a block diagram of an example of the SONET. The SONET shown in FIG. 4 includes transmission devices 10A, 10B, 10C and 10D, each of which has a higher bit rate of the bit rates of other transmission devices provided in the SONET. The transmission devices 10A-10D are coupled by means of optical fiber cables 11.sub.1 and 11.sub.2 in a dual loop (ring) formation. Transmission devices having bit rates equal to or lower than the transmission devices 10A-10D can be coupled to the transmission devices 10A-10D. For example, transmission devices 12a, 12b, 12c, 12d, . . . are connected to the transmission device 10A. The transmission device 10A multiplexes signals transmitted from the transmission devices 12a, 12b, 12c, 12d and so on via optical fiber cables 13a, 13b, 13c, 13d and so on. Then, the transmission device 10A sends a resultant multiplexed signal to either the transmission device 10B or 10D or both thereof. For the sake of convenience, the terms "east" and "west" can be used to describe the directions in which the signals are transferred. In FIG. 4, the transmission device 10D is located at the east side of the transmission device 10A, and the transmission device 10B is located at the west side thereof.
Although not shown in FIG. 4, transmission devices having bit rates lower than those of the transmission devices 12a, 12b, 12c and 12d are coupled thereto via optical fiber cables. That is, the system shown in FIG. 4 has a hierarchical structure in which signals from various terminals such as telephone sets, facsimile machines and personal computers are sequentially multiplexed in accordance with the given hierarchy, and the multiplexed light signals are transferred via the transmission devices 10A-10D. In practice, the transmission devices 10B and 10D may be repeaters (regenerators).
FIG. 5 is a block diagram of the transmission device 10A shown in FIG. 4. The transmission device 10A includes a plurality of line termination parts 21.sub.1, 21.sub.2, . . . , 21.sub.n (n is an arbitrary integer), a multiplexer/demultiplexer (MUX/DMUX) 22, a time slot assignment part 23 (hereinafter, simply referred to as a TSA part), a DCC relay/broadcast part 24 and a CPU 25. The working side optical carriers OC-N(W) of the line termination unit 21.sub.1 are connected to the optical fiber cables 11.sub.1 and 11.sub.2 in the east direction. The protection side optical carriers OC-N(P) of the lien termination unit 21.sub.1 are connected to the optical fiber cables 1 and 11.sub.2 in the west direction. The line termination part 21.sub.2 is connected to the optical fiber cable 13a (which is illustrated as a single line for the sake of convenience in FIG. 4). The optical fiber cables 11.sub.1 and 11.sub.2 carry, for example, the light signals OC-48, and the optical fiber cable 13a carries the light signal OC-12.
Each of the line termination parts 21.sub.1 -21.sub.n is equipped with a line terminator 25w on the working line side, a line terminator 25p, and an overhead terminator 26. Each of the line terminators 25w and 25p has the function of terminating the overheads, that is, the function of adding the overheads to the signals to be transmitted and dropping the overhead from the received signals. More particularly, the line terminators 25w and 25p performs line termination processes which include an opto-electric/electro-optical conversion process, a scramble/descramble process, and an overhead add/drop process. The line terminators 25w and 25p output data (from which the overhead has been dropped) to the multiplexer/demultiplexer 22. The line terminator 25w and 25p add the overhead to data received from the multiplexer/demultiplexer 22. The overhead terminator 26 performs an overhead terminating process in which the overhead received from the line terminator 25w or 25p is segmented into byte-based data to thereby produce overhead bytes, and the overhead bytes are output to the line terminator 25w or 25p and are added to a signal to be transmitted.
The multiplexer/demultiplexer 22 has a demultiplexing function of demultiplexing the data received from the line termination parts 21.sub.1 -21.sub.n into resultant signals STS-1, and a multiplexing function of multiplexing the signal STS-1 from the TSA part 23 to thereby produce the STS-N (corresponding to the light signal OC-N). For example, when the line termination part 21.sub.1 can process the signal STS-48, the multiplexer/demultiplexer 22 demultiplexes the signal STS-48 into 48 signals STS-1, and multiplexes 48 signals STS-1 into one signal STS-48.
The TSA part 23 performs a time slot assignment process in which the positions of the time slots of the signals STS-1 are assigned. For example, the TSA part 23 assigns the positions of the time slots of the 48 signals derived from the signal STS-48 to the line via which the data should be transmitted.
The DCC relay/broadcast part 24 extracts control data necessary to perform the relay/broadcast process from the overhead terminators 26 of the line termination parts 21.sub.1 -21.sub.n, and performs a given process for the extracted control data. The control data corresponds to data transmitted via data communication channels D1-D12 shown in FIG. 3. Hereinafter, the above control data will sometimes be referred to as DCC data. The data communication channels D1-D12 are used to transfer data between maintenance persons. More particularly, the data communication channels D1-D3 are used for a communication in the section, and the data communication channels D3-D12 are used for a communication in the line. The DCC relay/broadcast part 24 preforms a given relay/broadcast process when the DCC data from the optical fiber cable 11.sub.1 is relayed to some optical fiber cables or all of the optical fiber cables.
The above-mentioned parts shown in FIG. 5 are controlled by the CPU 25 connected thereto through a CPU bus 26.
FIG. 6 is a block diagram of the line terminators 25w and 25p. The following description assumes that the structure shown in FIG. 6 is the line terminator 25w. The line terminator 25w includes an opto-electric signal converter (O/E) 31, a descrambler (DSCR) 32, a framer circuit (frame synchronizing circuit) 33, an overhead byte drop part 34, a signal demultiplexer (DMUX) 35, an electro-optical signal converter (E/O) 36, a scrambler (SCR) 37, a frame pulse generator (PG) 38, an overhead byte add part 39, and a signal multiplexer (MUX) 40.
The opto-electric signal converter 31 converts a light signal received via an optical fiber cable (which is, for example, the east side optical fiber cable 11.sub.1) into an electric signal. The descrambler 32 descrambles the electric signal form the converter 31. The opto-electric signal converter 31 extracts a clock signal CLK from the converted electric signal, and sends the extracted clock signal CLK to the framer circuit 33. The framer circuit 33 produces a frame synchronizing signal from a descrambled signal (which is indicated as DATA in FIG. 6) and the clock signal CLK. The frame synchronizing signal indicates one frame, which corresponds to nine lines each consisting of 90 octets in the case of the signal STS-1. The frame synchronizing signal thus produced is applied to the overhead byte drop part 34 and the signal demultiplexer 35. The overhead byte drop part 34 separates the overhead and data from the above signal DATA. The overhead (DROPOHB) thus separated is output to the overhead terminator 26. The signal demultiplexer 35 demultiplexes, on the frame basis, the data supplied from the overhead byte drop part 34 in synchronism with the frame synchronizing signal. The dropped data is then output to the TSA part 22 via the multiplexer/demultiplexer 22. The above relates to a receive system of the line terminator 25w.
A transmit system of the line terminator 25w operates in synchronism with a master clock MCLK, which has the same frequency as that of the clock signal CLK extracted in the receive system. The frame pulse generator 38 generates a frame pulse from the master clock signal MCLK, and outputs the frame pulse to the overhead byte add part 39 and the signal multiplexer 40. The signal multiplexer 40 multiplexes data (ADD Data) received from the TSA part 22 via the multiplexer/demultiplexer 22 on the frame basis. The overhead byte add part 39 adds the overhead (ADD OHB) to the data multiplexed on the frame basis. In FIG. 6, the output signal of the overhead byte add part 39 is indicated as DATA. The descrambler 37 descrambles the signal DATA. The electro-optical signal converter 36 converts the scrambled signal from the scrambler 37 into a light signal, which is then output to the light fiber cable.
FIG. 7 is a block diagram of an internal structure of the overhead terminator 26 shown in FIG. 5. The overhead byte (DROP OHB) from the line terminator shown in FIG. 14 is processed by an input system, which is made up of a receive buffer (REC buf) 41, a receive frame pulse generator (RPG) 42, an overhead byte demultiplexer (DMUX) 43, an overhead byte receive register (INF-R) 44, and an overhead byte receive serial port (S-PORTR) 45. The overhead byte (ADD OHB) output to the line terminator is processed by an output system, which is made up of an overhead byte multiplexer 48, an overhead byte transmit register (INF-S) 49, and an overhead byte transmit serial port (S-PORTS) 50.
The overhead (DROP OHB) from the overhead terminator 26 is temporarily stored in the receive buffer 41, and is then applied to the overhead demultiplexer 43. The overhead demultiplexer 43 uses the frame pulse output by the receive frame pulse generator 42, and demultiplexes the received overhead on the byte basis. The overhead bytes thus demultiplexed are output to the receive register 44 and the receive serial port 45. The overhead bytes stored in the receive register 44 are required to be processed by the CPU 25 shown in FIG. 5. Examples of these overhead bytes are bytes K1 and K2, which forms an automatic protection switching (APS). The receive serial port 45 includes a plurality of serial ports, via which the overhead bytes (OHBR) other than those to be processed by the CPU 25 are output on the port basis. For example, the overhead bytes E1 and E2 for a speech communication are connected to a speech codec (not shown). The DCC data to be relayed or broadcasted is output to the DCC relay/broadcast part 24 via the receive serial port 45.
The overhead bytes transferred via the CPU bus 26 are temporarily stored in the transmit register 49 of the transmit system, and are then output to the overhead multiplexer 48. The overhead bytes (OHBS) from the serial ports (not shown) and the overhead bytes DCC from the DCC relay/broadcast part 24 are output to the overhead byte multiplexer 48 via the transmit serial port 50. The overhead byte multiplexer 48 multiplexes the received overhead bytes in accordance with a transmit frame pulse generated from the master clock MCLK by the transmit frame pulse generator 47. Then, the multiplexer 48 outputs the multiplexed overhead to the signal multiplexer 40 shown in FIG. 6 via the output buffer 46.
FIG. 8 is a block diagram of an internal structure of the DCC relay/broadcast part 24. The DCC data among the overhead bytes OHBR from the receive serial ports 45 of the overhead byte terminator 26 shown in FIG. 7 is applied, for each line, to a protocol terminator 53 via a receive buffer (REC buf) 51 in synchronism with the operation clock of the CPU 25 shown in FIG. 5. The protocol terminator 53 operates in synchronism with the CPU operation clock. The overhead bytes DCC output by the protocol terminator 53 is output to a transmit serial port 50 of each of the overhead bytes terminators 26 via an output buffer (OUT buf) 52. The protocol terminator 53 can be formed of a microprocessor or the like, and terminates the overhead bytes DCC. If the overhead bytes DCC includes information indicative of a request for repeat or broadcast, the protocol terminator 53 outputs the received overhead bytes DCC to the corresponding overhead byte terminator(s) 26 via the output buffer 52 (relay/broadcast process).
However, the above conventional transmission device has the following disadvantages.
The overheads are terminated by the overhead byte terminators 26 respectively provided in the line termination parts 21.sub.1 through 21.sub.n. Since the line termination parts 21.sub.1 through 21.sub.n are respectively equipped with the overhead byte terminators 26, and have a large load and a large scale size.
The overhead bytes DCC necessary for the relay/broadcast are gathered in the DCC relay/broadcast part 24 and are then terminated. That is, the overheads are diassembled every byte, and the overhead bytes thus obtained are gathered in the DCC relay/broadcast part 24 for the relay/broadcast process. Hence, as an increased number of line termination parts is used, an increased number of signal lines used to gather the overhead bytes is needed and a more complex connection of the signal lines is required. Further, if an overhead byte to be relayed or broadcasted should be newly added, it is necessary to change the setting of the line termination parts 21.sub.1 through 21.sub.n and the DCC relay/broadcast part 24. For example, new buffers should be provided in addition to the buffers 51 and 52 shown in FIG. 8, and the setting of the protocol terminator 53 should be changed.